Cathodic electrodepositable resin compositions are well known in the art. The resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a primary nitrogen. The nitrogen is typically introduced through reaction with an amine compound. The cathodic electrocoat resins are typically blended with a crosslinking agent and salted with an acid to form a water emulsion, commonly referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, water, coalescent solvents, antifoam agents, etc., at the coating site to form an electrodeposition bath. The electrodeposition bath is typically contained in an electrically insulated tank containing an anode. The object to be coated comprises an electrically conductive material. The object is connected to a direct current circuit and acts as a cathode when immersed in the tank in combination with the previously mentioned anode. The flow of current through the object results in an electromotive force being exerted upon the principal resin and the pigment paste resulting in the deposition of a layer of resin and pigment paste upon the object. The thickness of the layer is a function of the principal emulsion, the pigment paste, the bath components, the electrical operating characteristics, the immersion time, etc.
The coated object is removed from the bath once the desired film thickness is obtained. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking.
Cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 3,984,299; 4,093,594; 4,134,864; 4,137,140; 4,419,467 and 4,468,307, the disclosures of which are incorporated by reference.
It is critical that the electrodeposited coating be smooth, have uniform thickness, and be free of defects such as craters, orange peel, etc., since the electrodeposited coating will be typically coated with a finish paint or topcoat (e.g., a pigmented base coat and a clear topcoat). It is known that the smoothness of a coating is related to the presence of coalescent solvents in the coating bath. Coalescent solvents are volatile organic compounds such as ethylene glycol monomethylether, ethylene glycol monobutylether, diethylene glycol monobutylether, ethanol, isopropanol, ethylene glycol monohexylether, propylene glycol monophenylether, Synfac.TM., and PCP.TM.. The coalescent solvents are typically present in a coating bath in a range of about 2.5 wt. % to about 5.0 wt. %, typically about 2.9 wt. %, wherein the bath has a concentration of about 20 wt. % to about 25 wt. % solids. It is known that the absence of coalescent solvents in a coating bath will result in electrodeposited coatings which are rough, have nonuniform thickness, have craters, and have low rupture voltage. It is theorized that this occurs since the coalescent solvents act as plasticizers to lower the Tg of the system. Tg is defined as a glass transition temperature. In addition, it is thought that the coalescent solvents help the dispersed particles in the coating bath to deposit uniformly.
There is a constant search in the coatings industry for low VOC (Volatile Organic Content) coatings. This is particularly true in high volume, high use production operations such as automobile assembly plants. In order to meet government emission regulations and protect the safety and health of the workers, it is desirable to eliminate the VOC of coatings such as electrodeposited cationic coatings.
Accordingly, what is needed in this art is a principal emulsion and an electrodeposition bath in which coalescent solvents are substantially reduced or eliminated.